Recently, there has been growing interest in energy storage technologies. As the application fields of energy storage technologies have been extended to mobile phones, camcorders, notebook computers and even electric cars, efforts have increasingly been made towards the research and development of electrochemical devices. In this aspect, electrochemical devices have attracted the most attention. The development of secondary batteries capable of repeatedly charging and discharging has been the focus of particular interest. In recent years, extensive research and development has been conducted to design new electrodes and batteries for the purpose of improving capacity density and specific energy of the batteries.
Many secondary batteries are currently available. Lithium secondary batteries developed in the early 1990's have received a great deal of attention due to their advantages of higher operating voltages and much higher energy densities than conventional batteries using aqueous electrolyte solutions, such as Ni-MH batteries, Ni—Cd batteries and H2SO4—Pb batteries.
Generally, a secondary battery is fabricated by laminating or winding one or more unit cells, each of which includes a cathode, an anode and a separator interposed between the anode and the cathode, accommodating the laminated or wound unit cells in a metal can or a laminate sheet case, and injecting or impregnating an electrolyte solution thereinto.
Constituent electrode assemblies of secondary batteries have a cathode/separator/anode structure and are broadly classified into jelly-roll (i.e. winding) and stack (i.e. laminate) types by the constructions they have. A folding (jelly-roll) type electrode assembly is constructed by interposing a separator between a cathode and an anode, each of which is in the form of a long sheet to which an active material is applied, and winding the electrode structure. A stack type electrode assembly is constructed by sequentially laminating a plurality of cathodes and anodes, each of which has a predetermined size, between which separators are interposed. The jelly-roll type electrode assembly is easy to construct and has an advantage of high energy density per unit weight.
Porous polyolefin substrates are commonly used as separators of lithium secondary batteries including jelly-roll type electrode assemblies. Porous polyolefin substrates tend to undergo extreme thermal shrinkage at temperatures of 100° C. or higher due to their material characteristics and production processes including elongation. Under such circumstances, the introduction of an organic-inorganic porous coating layer into a separator has been suggested as an approach aimed at achieving improved heat resistance. However, when a mandrel is withdrawn or the separator is cut after winding in the course of the fabrication of an electrochemical device, defects may be caused. For example, the inorganic particles may fall off from the organic-inorganic porous coating layer or the organic-inorganic porous coating layer may be separated from the separator.